1. Field of the Invention
This invention relates to a semiconductor device which is operable at a high speed of several hundreds MHz or more.
2. Description of the Related Art
FIG. 1 is a perspective view, showing a conventional semiconductor device, and FIG. 2 is a sectional view, showing an essential part of the semiconductor device of FIG. 1. As is shown in FIGS. 1 and 2, a semiconductor device 1 comprises a semiconductor chip 2, a base portion 3 and a sealing portion 4 which seal the chip 2 therebetween. The base portion 3 and the sealing portion 4 are formed of ceramic which has superior high-speed operability, radiation properties and electric characteristics.
Specifically, the semiconductor chip 2 is bonded to a center portion of the lower surface of the base portion 3, with a paste 5 interposed therebetween. An iron-based lead frame 7 is bonded to a peripheral portion of the lower surface of the base portion 3 with the use of a glass-based adhesive 6 (42 alloy). An end of the lead frame 7 is electrically connected to the semiconductor chip 2 by means for a bonding wire 8. A peripheral portion of the sealing portion 4 is attached to the lower surface of the lead frame 7 by means of the adhesive 6. The sealing portion 4 seals the semiconductor chip 2, the bonding wire 8 and part of the lead frame 7.
"42 alloy", which is the material of the lead frame 7, has an expansion coefficient substantially the same as the glass-based adhesive 6. However, this alloy has inferior electric characteristics, and hence cannot be used in a semiconductor device which is to be operated at high speed. Specifically, "42 alloy" has high resistance and magnetism, and therefore its inductance changes depending upon frequency. As a result, in a higher frequency band, the output waveform is distorted as compared with the input waveform. This being so, a 42-alloy lead frame cannot be used in a semiconductor device which must be operated at a high speed.
The above problem can be solved by using, as the material of the lead frame, Cu having superior electric characteristics. In this case, however, cracks may occur in the glass-based adhesive 6 due to a difference between the expansion coefficients of the adhesive 6 and Cu, degrading the reliability of the semiconductor device.